A galvanizing installation comprises, in a general way, a bath of molten metal such as zinc through which the metal strip is continuously fed. A the output of the bath, the strip is covered, on both of its faces, by a metal deposit. It is necessary to regulate the thickness of the metal layer thus deposited, and, to this end, use is generally made, on at least one face of the strip, of an air knife regulating device which is used to blow a flat jet of fluid onto the strip in a direction substantially perpendicular to it. To produce a flat jet, use is generally made of a blower nozzle supplied with pressurized liquid and provided with a rectangular opening defined by two lips spaced a small distance apart and extending across the entire width of the strip. The thickness of the coating depends on the distance between the nozzle and the strip and on the pressure exerted by the jet, it being possible to regulate the latter by modifying the spacing of the lips.
The geometry of the line and the conditions for implementing the process, such as the zinc solidification method, and the stresses of the strip cause the sheet to vibrate at relatively erratic amplitude and frequency. These vibrations, whose amplitudes can exceed ten millimeters, result in variations in thickness which are incompatible with quality requirements, and additional costs due to the overconsumption of zinc.
This metal being, generally, expensive, it is desirable to limit the thickness of the deposit to the desired value.
To reduce the vibrations, one approach would be to act on the traction of the strip or on the arrangement of the rollers, or to place dampening means, for example of the pneumatic or magnetic kind, along the sheet. Such means, however, complicate the installation and are, in any case, relatively expensive.
It has also been proposed to stabilize the strip leaving the bath by means of two fluid cushions by providing, on each side of the strip and above the nozzles, a flat space defined by a flat wall parallel to the strip and in which a fluid is injected under pressure which then escapes from the two ends of the space and forms, on the upstream side, an overpressure zone. Such a device must, however, be spaced a certain distance from the nozzles, which reduces its effectiveness. Moreover, it is difficult to correctly adjust the pressure of the fluids injected on either side of the strip.
It has also been proposed in (JP-A-54.93638) to extend the upper lip of the nozzle, on the downstream side in relation to the strip feed direction, by a guide wall for guiding the fluid blown by the nozzle, so as to form a fluid cushion. The wall extending along the strip is corrugated so as to form successive zones in which the pressure is progressively reduced. However, turbulence may form above the upper lip causing a depression that can lead to the strip sticking to the upper lip.
The arrangements known to date for dampening the vibrations were therefore relatively complex and expensive and difficult to implement and, moreover, it was difficult to adjust the pressures in the fluid cushions to the entire satisfaction of users.